1. Field of the Invention
This invention relates in general to the fabrication of optical surfaces for large mirrors and, in particular, to the fabrication of spherical and aspherical optical surfaces using computer-controlled lapping processes.
2. Description of the Prior Art
The use of large mirrors for astronomical telescopes and for optical systems involved in surveillance systems is well known in the prior art. Historically, large mirrors used for astronomical applications have been manufactured from large blank of glass with the reflecting surface supported by a thick substrate to insure that the shape of the reflecting surface was accurately maintained. As requirements evolved for increasingly larger surface area mirrors, the use of the well known expedient of increasing the thickness of the mirror blank to insure stability of the reflecting surface became less desirable, due to the substantial increase in weight of the mirror structure. Recent advances have resulted in a need for large-diameter mirrors for both astronomical and surveillance applications, with the applications frequently requiring that the mirrors be installed and operated in systems located in earth orbit. The requirement of transporting the mirror from the earth's surface into earth orbit mandates that every effort be made to reduce the weight of the mirror structure, while maintaining the rigidity of the mirror to insure absolute tolerance in the geometry of the mirror's reflecting surface. Due to the difficulty of handling and fabricating large blanks of glass or other material into reflecting surfaces, techniques have been developed for producing light-weight mirror structures which result in relatively rigid reflecting surfaces having high optical tolerances while reducing the overall weight of the mirror. These techniques include the use of machining to reduce the weight of a monolithic blank from which a mirror is fabricated, and the construction of mirrors having large surface areas by utilizing multiple mirror segments, each of which is precisely aligned with adjoining segments to form a large reflecting surface. Mirrors constructed in accordance with the foregoing arrangements utilize mirror components in which the support for the reflecting surface is obtained from a series of support webs which support a relatively thin facesheet which forms the mirror's reflecting surface. Each mirror blank is carefully fabricated so that it may be joined to other blanks to form a reflecting surface whose surface area is substantially larger than that of the individual blanks from which the mirror is fabricated.
In response to the recent development of fabricating large mirrors by assembling mirror segments into a monolithic mirror, attempts have been made to develop techniques for individually lapping the surface of each mirror segment to produce individual mirror sections having reflecting surfaces of a precise geometry across the entire surface which may then be joined together so that the assembled mirror has a reflecting surface with the desired surface geometry. As used herein, the term "lapping" is meant to include the separate processes of "grinding", in which particles of material of a relatively large size are removed and "polishing", in which smaller size particles of material are removed to produce the final optical surface. Such techniques have employed computer controlled machines to grind and to polish the reflecting surface of each segment to achieve the desired surface geometry for the mirror. While this technique is generally useful for producing mirror segments for reflecting surfaces having a large surface area, problems occur when the technique is attempted to be applied to mirror segments in which a high degree of light-weighting is to be achieved (i.e., where more than 80 percent of the weight of a mirror blank is to be removed) and where a web is used to support a very thin facesheet. In particular, attempts at grinding and polishing a thin facesheet with conventional surface grinding tools produces an effect known in the trade as "print-through". In addition, residual surface errors are produced around the edges of the mirror segment because of the difficulty of controlling the grinding and polishing tools near each edge of each mirror segment. Typically, a light weight mirror may have a facesheet for the reflecting surface which is as thin as five to ten millimeters thick which is supported by a rib structure to insure rigidity of the reflecting surface. While operations are being carried out to remove material from the facesheet to produce the desired optical surface geometry, the lapping tool is pressed against the facesheet to control the speed at which material is removed from the facesheet. However, due to the pressure being exerted against the thin facesheet, the facesheet is likely to deflect in places in which it is not directly supported by the rib structure. After the lapping tool is removed from the facesheet, the facesheet springs back to its strain-free condition in places in which it was not supported during the grinding operation by the rib structure and, consequently, the surface of the facesheet takes on a topography which is characterized by high spots which result from the facesheet "springing back" after removal of the pressure applied by the grinding tool. This pattern is referred to as "print-through". While the print-through effect may be reduced by designing the mirror with a facesheet as thick as possible, this solution adds undesirable weight to the mirror structure. An alternate solution for reducing print-through is to substantially reduce the pressure used for grinding and polishing the reflecting surface. Unfortunately, this approach results in substantial additional time being required to finish the reflecting surface to the required surface geometry, and results in substantial additional cost being incurred in manufacturing mirrors having large reflecting surface areas. Consequently, neither of the foregoing solutions to print-through is entirely acceptable.
Another defect encountered in the prior art approach to fabricating mirrors with large surfaces areas is the difficulty of maintaining precise control of the surface geometry of segments of a mirror around the edges of each segment. These errors are frequently referred to as "residual edge zone errors" and are caused by the inability to efficiently control the distribution of material being removed from the facesheet in the area of the edge of the facesheets. This error results from the excessive tool pressure at the edge of the segment and from the differential tool wear while the tool overhangs the segment's edge because of the need to effectively finish the optical surface along the edge. Obviously, when a segmented mirror is to be assembled from a number of segments, the problem of edge zone control becomes substantial due to the appreciable surface area of the mirror which the edges of each segment comprise.